1. Field of the Invention
The invention concerns a method and circuit for the automatic control of the speed of a DC motor by the control voltage of the motor.
2. Description of the Prior Art
A standard method for achieving this automatic control makes use of the negative feedback of a tachometric generatrix. This negative feedback is a quantity representing the speed of the DC motor to which it is coupled. This method which is costly and, in certain applications, unsuitable because of the amount of space taken up by the tachometric generatrix called generator in the following description, has been replaced by the control method known as the "motional negative feedback" method.
This second method uses a bridge assembly, as illustrated by the diagram of FIG. 1, enabling access to the back electromotive force of the DC motor itself. This figure shows the motor 1 having an armature resistance r constituting one of the values of the four resistors r, s, p, g of the bridge 2 set up between the ground 3 and the outlet of a power amplifier 4 with a gain G. Between the terminals 5 and 6 for measuring the bridge 2, there is a first summator 7, the output signal (a voltage) of which corresponds to the difference between the potentials of the two terminals 5 and 6, the potential of the terminal 6, called an intensity feedback, being deducted from that of the terminal 5, called a voltage feedback. This output voltage is carried by an automatic control channel 8 to the input of the amplifier 4 where, by means of a second summator 9, it is deducted from the control voltage U also admitted at the input of the amplifier 4.
Computations show that in accurately choosing the resistance values of the resistors s, p and g for a given motor with a known armature resistance and for an amplifier with a given gain, this assembly makes it possible to obtain a speed proportionate solely to the control voltage U of the motor in a ratio where the effects of friction are notably reduced. Furthermore, by this arrangement, the electro-mechanical time constant is also reduced. This is particularly valuable when a fast response is desired (damping of a closed-loop servo-mechanism).
The limits of this device lie in the variation of the armature resistance r during operation. At a given temperature, namely that corresponding to the setting of the motor in accordance with a technical specification, the value of r is known and it is possible to provide for the conditions enabling a speed response from the motor that is proportionate to the control voltage U. However, in certain applications, the range of the temperatures in which the motor is made to function is very wide (sometimes wider than 100.degree. C.), so much so that the value of r of the bridge 2 may vary broadly below, above or around its nominal value, leading to non-compliance with the condition resulting from the computation to obtain the linearity of the speed response of the motor. In certain cases, even computation shows that this variation may lead to an instability in the automatic control, and the motor may go into over-speed. To prevent this instability, we are led to choose resistance values of the bridge such that the performance characteristics of the system are deliberately reduced to provide for adequate safety at extreme temperatures.
The invention is designed to overcome these drawbacks by improving the automatic control and to improve the range of satisfactory operation of the system.